Background has a very important influence on point target operating range of infrared search and track system(IRST).
Based on the noise equation temperature difference(NETD) operating range model , a general noise parameter δ′ which
describes the influence of background and system noise on the operating range is presented, and a new operating range
model is founded including δ′. δ′ has clear meaning and gives explicit explanation for conventional detector-limited and
background-limited operating range model. The algorithm of δ′ based on the ratio of background region to target area is
proposed under natural sky background. The simulation shows that δ′ is valid for predicting point target operating range
of IRST.